This invention concerns improvements in or relating to VSTOVL (vertical short take off vertical landing) gas turbine jet engines. In particular it provides a dual mode turbofan engine which in a first mode produces conventional forward thrust by means of a rear jet-pipe, and in a second mode produces vertical thrust or lift.
On a conventional take-off and landing engine (CTOL) the low pressure compressor (LPC) is sized to suit forward flight thrust requirements. On the other hand, a vertical take-off and landing (VTOL) engine has a need to maintain a particular thrust balance, in the hover mode, between front and rear nozzles. Hitherto, this has been achieved by making the LPC larger (ie oversizing) and so providing increased mass flow (=thrust) for the front nozzles. A natural and undesirable consequence of this is that the engine is larger and heavier than an equivalent CTOL engine. This in turn has the effect of making the airframe larger, thus causing an increase in weight and increased cost of ownership. Further, the increased size and weight of the airframe makes it more difficult for the aircraft to operate in the stealth mode, which is an increasingly important requirement of modern military aircraft.
It is an object of the present invention to provide a turbofan gas turbine jet engine that is capable of both conventional forward thrust and vertical thrust without a significant increase in size or weight.